Layered thin film heater and method of fabrication

ABSTRACT

A method of forming thin film heater traces on a wafer chuck includes positioning a pattern, that forms openings corresponding to a desired layout of the heater traces, in proximity to the wafer chuck. The method includes sputtering a material toward the pattern and the wafer chuck such that a portion of the material passes through the openings and adheres to the wafer chuck to form the heater traces. A method of forming thin film heater traces on a wafer chuck includes sputtering a blanket layer of a material onto the wafer chuck, and patterning a photoresist layer utilizing photolithography. The photoresist layer covers the blanket layer in an intended layout of the heater traces, exposing the blanket layer in areas that are not part of the intended layout. The method removes the areas that are not part of the intended layout by etching, and removes the photoresist layer.

TECHNICAL FIELD

The present disclosure is directed to technology for fabricating heaterelements, as may be used, for example, in semiconductor waferprocessing. In particular, systems and methods of fabricating thin filmheater traces for wafer chucks are disclosed.

BACKGROUND

Semiconductor wafer processing is sometimes performed in batches, withall wafers in a cassette being subjected to a process simultaneously.However, single water processing is becoming more prevalent becausepoint to point process variations in a single water processing apparatuscan usually be controlled more precisely. In single wafer processing,wafers are usually moved by automated handlers from cassettes to a waterchuck and back again. In some cases, the wafer sits on the chuck, heldin place only by gravity. In other cases, the wafer is held either byapplying a vacuum to one or more backside areas of the water throughholes in the chuck (e.g., a “vacuum chuck”) or by applying a voltage toa chuck that is formed of an insulator overlying a conductor (e.g., an“electrostatic chuck” (ESC)). Processing may involve controlling thetemperature of the chuck and thus the wafer. Certain chucks have bothheating and cooling capability, while others have only heatingcapability. Chucks can be heated by running a heated fluid through thechuck or by passing current through a resistive heater elementintegrated with the chuck.

Heating capability for a wafer chuck is often implemented by formingresistive heater traces on the chuck. Presently, such heater traces areoften fabricated using thick film technologies such as screen printing,ink jet printing or other liquid dispensing techniques. In thesetechniques, forming precise lines and corners, as well as forming layerswith precise thickness control, can be challenging.

SUMMARY

The present disclosure is directed to formation of heater traces, formedby sputtering, on wafer chucks for processing equipment. Such heatertraces are expected to provide improved resistance control and thusimproved process control achieved by the processing equipment, comparedto prior art heater traces.

In an embodiment, a method of forming thin film heater traces on a waferchuck includes positioning a pattern in proximity to the wafer chuck,the pattern forming openings corresponding to a desired layout of theheater traces. The method also includes sputtering a material toward thepattern and the wafer chuck such that a portion of the material passesthrough the openings and adheres to the wafer chuck to form the heatertraces.

In an embodiment, a method of forming thin film heater traces on a waferchuck includes sputtering a material onto the wafer chuck to form ablanket layer of the material, and patterning a photoresist layerutilizing photolithography. The photoresist layer covers the blanketlayer in an intended pattern of the heater traces, while exposing theblanket layer in areas that are not part of the intended pattern. Themethod also includes removing the areas that are not part of theintended pattern by etching, and removing the photoresist layer.

In an embodiment, a heated wafer chuck for semiconductor processingincludes a wafer chuck having a dielectric surface, and one or moresputtered thin film heater traces. The one or more sputtered heatertraces include copper, molybdenum or alloys thereof. A layout of theheater traces includes landing pads for electrical connections. Adielectric layer overlies the one or more sputtered thin film heatertraces. The dielectric layer forms vias over the landing pads, for theelectrical connections to make contact to the landing pads.

Additional embodiments and features are set forth in part in thedescription that follows, and in part will become apparent to thoseskilled in the art upon examination of the specification or may belearned by the practice of the invention. The features and advantages ofthe invention may be realized and attained by means of theinstrumentalities, combinations, and methods described in thespecification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates major elements of a single wafersemiconductor processing system, according to an embodiment.

FIG. 2 schematically illustrates a single wafer chuck, according to anembodiment.

FIGS. 3A and 3B are schematic cross-sectional illustrations of twovariations of the single wafer chuck of FIG. 2, according toembodiments.

FIGS. 4, 5 and 6 are cross-sectional, schematic illustrations of methodsof forming thin film heater traces with sputtering, according toembodiments.

FIG. 7 shows thin film heater traces on a chuck, representing the resultof any of the methods illustrated in FIGS. 4, 5 and 6.

DETAILED DESCRIPTION

The present disclosure may be understood by reference to the followingdetailed description taken in conjunction with the drawings brieflydescribed below, wherein like reference numerals are used throughout theseveral drawings to refer to similar components. It is noted that, forpurposes of illustrative clarity, certain elements in the drawings maynot be drawn to scale. Specific instances of an item may be referred toby use of a numeral in parentheses (e.g., landing pads 220(1), 220(2))while numerals without parentheses refer to any such item (e.g., landingpads 220). In instances where multiple instances of an item are shown,only some of the instances may be labeled, for clarity of illustration.

FIG. 1 schematically illustrates major elements of a single wafer,semiconductor wafer processing system 100, according to an embodiment.Processing system 100 includes a housing 110 for a wafer interface 115,a user interface 120, a process chamber 130, a controller 140 and one ormore power supplies 150. Process chamber 130 includes one or more waferchucks 135, upon which wafer interface 115 can place a wafer 50 forprocessing. The elements shown as part of system 100 are listed by wayof example and are not exhaustive. Many other possible elements, suchas: pressure and/or flow controllers; electrodes, magnetic cores and/orother electromagnetic apparatus; mechanical, pressure, temperature,chemical, optical and/or electronic sensors; viewing and/or other accessports; and the like may also be included, but are not shown for clarityof illustration. Internal connections and cooperation of the elementsshown within system 100 are also not shown for clarity of illustration.Representative utilities such as gases 155, vacuum pumps 160, radiofrequency generators (RE Gen) 165 and/or electrical power 170 mayconnect with system 100. Like the elements shown in system 100, theutilities shown as connected with system 100 are intended asillustrative rather than exhaustive; other types of utilities such asheating or cooling fluids, pressurized air, data exchange (e.g.,networking or automation) capabilities, waste disposal systems and thelike may also be connected with system 100, but are not shown forclarity of illustration.

FIG. 2 schematically illustrates a single wafer chuck 200, according toan embodiment. Chuck 200 is suitable for use as chuck 135 shown inFIG. 1. Chuck 200 may be configured as an electrostatic chuck or avacuum chuck to hold wafer 50 for processing, according to structuredescribed below in connection with FIGS. 3A and 3B. Chuck 200 includes athin film heater trace 210 that is formed by sputtering. The shape ofthin film heater trace 210 shown in FIG. 2 is for illustrative purposesonly and may vary, in embodiments, in order to provide adequate and/oruniform heating of wafer 50. In particular, embodiments herein mayinclude multiple thin film heater traces 210 that heat differing radialand/or lateral zones of chuck 200. Multiple thin film heater traces maybe used to compensate for any nonuniformity in temperature that mayarise, due to minor asymmetry and/or external reasons such as transfersof heat among chuck 200 and wafers in process, plasmas, processchemicals and/or other parts of equipment in which chuck 200 resides.Thin film heater trace 210 terminates in landing pads 220(1) and 220(2)that may connect by electrical connections to a power supply (e.g., oneof power supplies 150 shown in FIG. 1) that provides current for heatingchuck 200.

Chuck 200 may be formed of an insulator, for example ceramic, withheater trace 210 formed directly thereon. Exemplary insulators of whichchuck 200 may be formed include alumina (Al₂O₃) and aluminum nitride(AlN). Alternatively, chuck 200 may be formed of a conductor overlaidwith a dielectric layer with heater trace 210 formed on an opposite sideof the dielectric layer from the conductor. Exemplary conductors ofwhich chuck 200 may be formed include brass, stainless steel andaluminum; exemplary dielectric layers include silicon dioxide (SiO₂),silicon nitride (Si₃N₄) and mixtures thereof (Si_(x)O_(y)N_(z)). Thinfilm heater trace 210 may be formed of copper, molybdenum, alloysthereof, and/or other materials that are typically utilized for heatertraces.

Chuck 200 may optionally include one or more sensors 230 (for example,thermocouples) to provide temperature measurements of chuck 200. Sensors230 may, in embodiments, be embedded within channels formed in chuck200. Although only one sensor 230 is shown in FIG. 2, it is understoodthat multiple sensors 230 may be disposed at various locations toprovide temperature measurements at each of the locations. In operation,a power supply (e.g., one of power supplies 150 shown in FIG. 1)supplies power through electrical connections to thin film heater trace210, heating chuck 200 to an appropriate temperature for waferprocessing. Temperature measured by sensors 230 and/or electrical powerdelivered to heater trace 210 may be measured and reported continuouslyor intermittently to an operator of equipment that includes chuck 200. Abroken line 3-3′ shows a plane corresponding to the schematic viewsshown in FIGS. 3A, 3B.

The choice of sputtering as a method for forming the material of heatertrace 210 enables significant improvements. Existing heater traces forwafer chucks are often formed by thick film fabrication methods (e.g.,screen printing) to minimize cost. Such existing heater traces can havecompositional, thickness and/or lateral dimensional variations, andinstabilities over time and use that can adversely affect therepeatability and point to point uniformity of the heater traces. In anexample of such effects, wafer processing uniformity requirements placetight specification tolerances on resistance of heater traces and powerdelivery required to achieve a given wafer chuck temperature. Nativevariation of as-fabricated resistance measurements of thin film heatersmade with thin film fabrication results in some wafer chucks whoseheater traces exceed the resistance tolerances and must be scrappedbefore installation, while the wafer chucks that are within tolerancestend to fill the specification “window.” Furthermore, over time, eventhe wafer chucks that are within tolerances generate a wide distributionof power delivery required to achieve a given wafer chuck temperature.

Sputtering can be expensive in terms of capital equipment cost and perunit processed, but may provide more precise thickness and compositionalcontrol than thick film techniques, resulting in both cost savings andimproved process control of the finished product. Sputtering is alsocompatible with patterning techniques that can minimize lateraldimensional variations, as discussed below. It is expected that thinfilm heater traces manufactured by sputtering according to thetechniques below will provide tighter distributions of resistance andpower delivery required to achieve a given wafer chuck temperature. Forexample, it is expected that all wafer chucks manufactured will bewithin resistance specifications, thus reducing cost by reducing oreliminating out-of-tolerance wafer chucks, and reducing resistancevariation in the overall distribution of wafer chucks. The reducedresistance variation is expected to, in turn, reduce power deliveryvariation required to achieve a given wafer chuck temperature, thusimproving process uniformity of the equipment utilizing such waferchucks.

FIGS. 3A and 3B are schematic cross-sectional illustrations of twoversions of chuck 200, denoted as 200(1) and 200(2) respectively. InFIG. 3A, chuck 200(1) is an electrostatic chuck that includes aconductive base 240. Chuck 200(1) is formed by forming a first insulator250 upon base 240, sputtering thin film heater trace 210, and finallyforming a second insulator 260 over both first insulator 250 and thinfilm heater trace 210. First and second insulators 250 and 260 typicallyprovide mechanical integrity, chemical passivation and/or scratchprotection for an upper surface of chuck 200(1) and for heater trace210. The vertical dimension of thin film heater trace 210 is exaggeratedfor illustrative clarity. Vias are provided in second insulator 260(outside of the cross-sectional plane shown in FIG. 3A) to facilitateelectrical contact to heater trace 210, and are typically formed by amasking and etching process.

In FIG. 3B, chuck 200(2) is a vacuum chuck that includes a base 270formed of an insulator, and containing vacuum passages 290 thatoptionally connect with a vacuum connector 295 for further connection toa vacuum source. Chuck 200(2) is formed by forming vacuum passages 290in base 270, sputtering thin film heater trace 210, forming an insulator280 over heater trace 210, and completing vacuum passages 290 throughinsulator 280 such that vacuum can be applied to a backside of wafer 50to hold wafer 50 in place. Vacuum passages 290 may intersect a surfaceof chuck 200(2) as shown, or may intersect rings or other surfacestructure of chuck 200(2) to engage a greater backside area of wafer 50,to hold wafer 50 securely to chuck 200(2). Insulator 280 typicallyprovides mechanical integrity, chemical passivation and/or scratchprotection for an upper surface of chuck 200(2) and for thin film heatertrace 210. Again, the vertical dimension of thin film heater trace 210is exaggerated for illustrative clarity, and vias are provided in secondinsulator 260 (outside of the cross-sectional plane shown in FIG. 3B) tofacilitate electrical contact to heater trace 210.

FIGS. 4, 5 and 6 are cross-sectional, schematic illustrations of methodsof forming thin film heater traces with sputtering, according toembodiments. FIG. 7 shows thin film heater traces 390 on a chuck 300,representing the result of any of the methods illustrated in FIG. 4, 5or 6. It is understood that the shown geometries of heater traces 390are for purposes of illustration only and may vary in embodiments. Also,chuck 300 is represented only as a single block but may have structuresuch as insulator layers, vacuum channels, channels for sensors and thelike that do not interfere with the processes illustrated. Furthermore,upon reaching the condition illustrated in FIG. 7, further processes maybe implemented (e.g., deposition of further dielectrics, etc.)

1. Exemplary Stencil Mask Technique

FIG. 4 shows beginnings of thin film heater traces 390 being formed bysputtering metal atoms 340 through a stencil mask 320, wherein openingsin stencil mask 320 correspond to a desired layout of heater traces 390.Atoms 340 may be, for example, copper, molybdenum, alloys thereof,and/or other materials that are typically utilized for heater traces.Fixturing 330 holds stencil mask 320 in registration with chuck 300 sothat thin film heater traces 390 form in desired locations on chuck 300.Atoms 340 that encounter openings in stencil mask 320 simply passthrough stencil mask 320 and adhere to chuck 300, building up heatertraces 390. Atoms 340 that encounter stencil mask 320 in areas that arenot openings adhere to stencil mask 320. Sputtering of atoms 340 ceaseswhen heater traces 390 reach their desired thickness. Thereafter, chuck300 disengages from fixturing 330 such that stencil mask 320 and thesputtered atoms that have adhered thereto are removed.

2. Exemplary Photolithography Techniques

2A. Blanket Layer Deposition with Photolithography and Etch

FIG. 5 shows thin film heater traces 390 being formed by etching of ametal layer 350 that is first sputtered onto chuck 300 as a blanketlayer, and is subsequently patterned using photolithography. Aphotoresist layer is applied and developed to yield a photoresistpattern 360 corresponding to a desired layout of heater traces 390.Exposure and development steps of photoresist pattern 360 are not shown.Either positive or negative photoresist and mask polarity may beutilized to generate pattern 360. For example, to use positivephotoresist patterning, chuck 300 with a blanket layer 350 is firstcoated with positive photoresist, then exposed using a mask that blockslight exposure of areas intended as the heater traces. In positivephotoresist, light breaks down the photoresist, allowing a developmentstep to remove the photoresist that was exposed, leaving pattern 360 asillustrated in FIG. 5. Alternatively, to use negative photoresistpatterning, chuck 300 with blanket layer 350 is coated with photoresist,then exposed using a mask that allows light exposure of areas intendedas the heater traces. Light causes crosslinking of negative photoresistmolecules, allowing the exposed areas to remain after a development stepremoves photoresist that was not exposed, leaving pattern 360 asillustrated in FIG. 5.

Once pattern 360 is generated, metal layer 350 that is exposed throughopenings in pattern 360 are etched by an etchant 370; FIG. 5 illustrateslayer 350 in a partially etched state. Etchant 370 may be any etchantthat etches layer 350 suitably and is compatible with material(s) ofchuck 300 (e.g., etchant 370 may be chosen to etch layer 350preferentially with respect to material(s) of chuck 300, but someetching of chuck 300 may be acceptable).

Completion of etching and removal of photoresist pattern 360 results indefinition of one or more individual heater traces 390 in theconfiguration shown in FIG. 7.

2B. Photolithography Defining Regions for Selective Deposition, withLiftoff

FIG. 6 shows thin film heater traces 390 being formed by sputtering ofatoms 340 onto chuck 300 that is patterned in an inverse photoresistimage 380 of heater traces 390. Inverse image 380 is formed usingphotolithography, with either positive or negative photoresist and maskpolarity, as described above. After inverse image 380 is formed, metalatoms 340 are sputtered in the direction of chuck 300. Atoms thatencounter inverse photoresist image 380 adhere thereto, while atoms thatpass through openings in image 380 adhere instead to chuck 300, formingheater traces 390. Sputtering of atoms 340 ceases when heater traces 390reach their desired thickness. Thereafter, inverse photoresist image 380and the sputtered atoms that have adhered thereto are removed. Thetechniques illustrated in FIGS. 4 and 6 can be considered similar inthat each involves positioning a pattern in proximity to a chuck andsputtering atoms through the pattern to form the thin film heatertraces.

Both the stencil mask technique illustrated in FIG. 4 and thephotolithography techniques illustrated in FIGS. 5 and 6 arecontrollable to provide lateral dimension reproducibility on the orderof a micron or less, while thick film techniques are typicallycontrollable to provide lateral dimension reproducibility only on theorder of tens of microns. Control of lateral dimensions is one principalsource of improved dimensional control underlying improved control ofresistance in thin film heater traces fabricated by sputtering. A secondsource of improved dimensional control is thickness control. Heatertraces manufactured using thick film technology can have point to pointvariations (e.g., from point to point within traces of a given waferchuck) on the order of tens of microns. Such variations translatedirectly to existence of hot spots (w)ere traces are thin) or cold spots(w)ere traces are thick) on the wafer chuck. Thin film heater tracesmanufactured using sputtering are expected to have point to pointvariations with a given wafer chuck that are only on the order of tenthsor hundredths of a micron, with corresponding reduction in the tendencyof the wafer chuck to have hot or cold spots. Also, typical currentpractice is to test resistance of heater traces as fabricated on a waferchuck before integrating the wafer chuck into apiece of processequipment; it is believed that improved resistance variation control inthe heater element as discussed above will enable removal of the testingstep from the process equipment manufacturing flow, for additional costsavings. Net resistance variation among heater traces made utilizingthick film technology is rarely less than five percent, while netresistance variation among thin film heater traces made utilizingsputtering is expected to be less than two percent.

Having described several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. Additionally, a number of well-known processes and elementshave not been described in order to avoid unnecessarily obscuring thepresent invention. Accordingly, the above description should not betaken as limiting the scope of the invention.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimits of that range is also specifically disclosed. Each smaller rangebetween any stated value or intervening value in a stated range and anyother stated or intervening value in that stated range is encompassed.The upper and lower limits of these smaller ranges may independently beincluded or excluded in the range, and each range where either, neitheror both limits are included in the smaller ranges is also encompassedwithin the invention, subject to any specifically excluded limit in thestated range. Where the stated range includes one or both of the limits,ranges excluding either or both of those included limits are alsoincluded.

As used herein and in the appended claims, the singular forms “a”, “an”,and “the” include plural referents unless the context dearly dictatesotherwise. Thus, for example, reference to “a process” includes aplurality of such processes and reference to “the wafer chuck” includesreference to one or more water chucks and equivalents thereof known tothose skilled in the art, and so forth.

Also, the words “comprise,” “comprising,” “include,” “including,” and“includes” when used in this specification and in the following claimsare intended to specify the presence of stated features, integers,components, or steps, but they do not preclude the presence or additionof one or more other features, integers, components, steps, acts, orgroups.

What is claimed is:
 1. A method of forming thin film heater traces on awafer chuck, comprising: positioning a pattern in proximity to the waferchuck, the pattern forming openings corresponding to a desired layout ofthe thin film heater traces, and sputtering a material toward thepattern and the wafer chuck such that a portion of the material passesthrough the openings and adheres to the wafer chuck to form the thinfilm heater traces; wherein: the wafer chuck comprises ceramic, and theceramic forms vacuum channels configured to secure a wafer against thewafer chuck in proximity to the thin film heater traces.
 2. The methodof claim 1, the material comprising one or more of copper, molybdenum,and alloys thereof.
 3. The method of claim 1, wherein the desired layoutincludes landing pads configured for electrical connections to the thinfilm heater traces.
 4. The method of claim 3, further comprisingdepositing a dielectric layer over the thin film heater traces.
 5. Themethod of claim 4, further comprising opening vias through thedielectric layer to provide access for the electrical connections to thelanding pads.
 6. The method of claim 1, the wafer chuck comprising aconductor with an unbroken dielectric layer thereon, and wherein thethin film heater traces form on an opposite side of the unbrokendielectric layer from the conductor.
 7. The method of claim 6, theunbroken dielectric layer comprising a first dielectric layer, andfurther comprising depositing a second dielectric layer over the thinfilm heater traces.
 8. The method of claim 7, wherein the desired layoutincludes landing pads configured for electrical connections to the thinfilm heater traces, and further comprising opening vias through thesecond dielectric layer to provide access for the electrical connectionsto the landing pads.
 9. The method of claim 1, wherein: the patterncomprises a stencil mask; positioning comprises engaging the stencilmask in proximity to the wafer chuck using a fixture; and furthercomprising disengaging the stencil mask.
 10. A method of forming thinfilm heater traces on a wafer chuck, comprising: positioning a patternin proximity to the wafer chuck, the pattern forming openingscorresponding to a desired layout of the thin film heater traces, andsputtering a material toward the pattern and the wafer chuck such that aportion of the material passes through the openings and adheres to thewafer chuck to form the thin film heater traces; wherein: the patterncomprises an inverse photoresist image formed by photolithography;positioning comprises forming the inverse photoresist image on the waferchuck; and further comprising removing the inverse photoresist image.11. A method of forming thin film heater traces on a wafer chuck,comprising: sputtering a material onto the wafer chuck to form a blanketlayer of the material; patterning a photoresist layer utilizingphotolithography, the photoresist layer covering the blanket layer in anintended layout of the thin film heater traces, while exposing theblanket layer in areas that are not part of the intended layout;removing the areas that are not part of the intended layout by etching;and removing the photoresist layer.